The invention relates to a data transmission method and radio system. In particular, the invention relates to a method and system in which both the GMSK and the m-PSK modulation method are available for the signal to be transmitted and in which the modulation method used at a given time can be changed to another modulation method.
In radio telecommunication systems the quality of the channel, i.e. the radio path, varies continuously. Several factors affect the quality of a radio system channel, for instance multipath propagation, fading, interference in the environment and many other matters.
When designing known radio systems, the aim has been to secure the quality of the signal also when the quality of the channel is poor. When designing data transmission systems, an essential parameter is the modulation method used in the transmission path. The information symbols to be transmitted cannot as such be transmitted over the transmission path due to the dissipation occurring in the transmission path and the capacity of the transmission path, and the symbols must be modulated by a suitable method to produce a satisfactory transmission path capacity and transmission quality. When designing known systems, the main focus in selecting the modulation method has been to secure the transmission quality making the performance of the modulation methods in poor channel conditions very important. This is why the ability of current systems to transmit signals having high data rate is rather poor. When securing the quality of the transmission, the capacity has suffered.
An example of known modulation methods is GMSK (Gaussian Minimum Shift Keying), which is used in the GSM (Global System for Mobile Communication) cellular radio system. It has a limited frequency spectrum and its performance is good, but the data transmission rates are not very high. The continuous phase shift keying methods m-PSK make it possible to attain high data rates, but the modulation method works well only when the transmission channel does not have much interference, i.e. the signal-to-noise ratio is good.
One solution for optimizing performance and transmission rate is to change the modulation method used according to current needs. The GMSK method can be used when good interference tolerance is required, and when the channel quality is good, for instance the 8-PSK method can be used to achieve three times as fast a data rate as compared with GMSK.
The problem with known radio systems is how to change the modulation method seamlessly during an ongoing connection. Change of modulation method causes problems especially in the receiver, because the transmitter can change its modulation method without notifying the receiver in advance. A seamless change of modulation method is, however, needed in packet switched data transmission, for instance.
Therefore, when using the GMSK and PSK methods, the receiver must when receiving a signal check and decide which of the modulation methods has been used in sending the signal. As the modulation method can change without prior notice, this checking of modulation method is a continuous operation, and the easier it is done, the better. Previously, the decision on the modulation method used was made on the basis of the training sequence of each frame. As the signal constellation of a received GMSK signal rotates in the receiver, the rotation must be removed before detection. The signal constellation of a PSK signal does not rotate in the receiver, so the signals must be treated in different ways before channel estimation. This causes problems, because before channel estimation the receiver does not know the modulation method used.
Thus, the object of the invention is to implement a method and a radio system implementing the method in a manner that the above-mentioned problems are solved. This way, the receiver can effectively demodulate and detect the transmitted signal even though it does not know the modulation method of the transmitter in advance. This is achieved by the data transmission method of the invention, in which method both the GMSK and the m-PSK modulation method are available for the signal to be transmitted and the modulation method used at a given time can be changed to another modulation method. In the method of the invention, when using the m-PSK modulation method, the signal is multiplied by a given coefficient making the received signal constellations similar regardless of the modulation method.
A further object of the invention is a radio system comprising a set of transmitters and receivers, of which transmitters at least some comprise both a GMSK modulator and a m-PSK modulator to modulate the signal and a control unit adapted to select the modulation method used at a given time. In the system of the invention, the m-PSK modulator of the transmitter comprises a multiplier adapted to multiply the signal to be transmitted by a given coefficient which makes the signal constellations of the received signal similar regardless of the modulation method.
The method and system of the invention provide several advantages. Different modulation methods can be seamlessly combined in the solution of the invention, because the signals transmitted by different modulation methods can be treated in the same way before channel estimation.
In a preferred embodiment of the invention, the m-PSK signal to be transmitted is multiplied by a coefficient which makes the signal constellation in the receiver rotate in the same way as the constellation of a GMSK-modulated signal. With the solution of the invention one disadvantage of the PSK modulation method is avoided, which occurs when the data to be transmitted contains only zero bits. In such a case, the modulated signal comprises only the carrier wave, but in the solution of the invention, this situation is avoided due to the rotation of the constellation.